Immunoassays are primarily used for the detection and quantitation of a wide variety of analytes such as hormones, tumor antigens, bacterial or viral antigens, protein therapeutics etc. in biological samples. Quantitative immunoassays are critical for understanding the Pharmacokinetic profile of the therapeutic proteins in support of efficacy and safety studies. In addition, immunoassays are also used to quantitate the soluble targets of the therapeutic proteins and anti-therapeutic protein antibodies. For soluble protein targets or shed receptor targets of the analyte, two different forms of the analyte exist in the biological matrix: the free analyte and the analyte complexed with the soluble target. In order to get an accurate pharmacokinetic profile of the analyte, it is ideal to develop ligand binding assay methods aimed at quantitating the total analyte concentrations instead of only the free analyte concentrations.
A frequent problem often encountered in quantitative immunoassays is accurate estimation of the total analyte concentrations in the presence of an interfering substance (soluble target) which competes with the immunoassay reagents for binding to the analyte. This leads to an underestimation of the analyte and compromises the true assessment of the pharmacokinetic profile of the analyte. The interfering substance can be removed by pre-treatment of the samples. However this step leads to loss of sample and introduces additional variability.